1. Field of the Invention
The present invention relates to a method of fabricating an optical fiber with longitudinal holes.
2. Description of the Prior Art
A fiber with holes conventionally consists of a solid dielectric material such as silica including a distribution of regularly spaced patterns known as “holes”. The holes are generally filled with air, but can consist of a dielectric material other than silica having a different, preferably lower, refractive index. The holes in a fiber take the form of tubes extending longitudinally along the fiber in the signal propagation direction. This kind of fiber structure is described in the paper “All silica single mode optical fiber with photonic crystal cladding” by J. C. Knight et al, Optical Letters, Vol. 21, No. 19, PP 1547-1549, Jan. 1, 1996.
In a fiber with holes, two light guiding mechanisms are used. One is guidance by total internal reflection, associated with the fact that the area including the holes has an index lower than that of silica, and the other is guidance by forbidden photonic bands, when the holes are disposed in accordance with a periodic structure and the periodicity of the structure is broken at the level of the core of the fiber.
The regular disposition of the holes in the silica makes this kind of structure similar to a crystal, referred to as a photonic crystal, whose characteristics are determined among other things by the spacing between the holes, i.e. the pitch, and by the rate of filling of the solid material by the holes, known as the air filling ratio, i.e. depends on the diameter of said holes.
Optical fibers with holes have beneficial chromatic dispersion, polarization maintenance and effective surface area properties, which makes them attractive as component fibers and as line fibers.
The fabrication of fibers with holes is based on a prior art technique of fabricating “microstructured” optical fiber. For example, European patent EP 0 810 453 B1 describes one method of fabricating this kind of fiber.
A silica rod forming a core is surrounded with capillary tubes and the resulting bundle is disposed in a preform. The fiber is then drawn from a heated end of the preform, the capillary tubes being sealed at the other end.
The geometry of the fiber with holes depends on the assembly of the capillary tubes around the central rod that forms the guiding part of the fiber. The preliminary fiber of the capillary tubes is therefore of primordial importance because it determines the diameter of the holes in the fiber, i.e. has a direct influence on the filling ratio as previously defined and thus on the guidance properties of the fiber obtained.
FIG. 1 is a diagrammatic view in section of a preform for producing a microstructured optical fiber.
The capillary tubes 12 are usually obtained by shrinking and drawing a tube, for example a silica or silicate tube, having an interior diameter tint and an exterior diameter φext having a specific ratio φint/φext. For example, capillary tubes can be shrunk on a glassmaking lathe using a torch or a furnace, and drawing is effected conventionally in a fiber drawing tower. Shrinking and drawing can also be carried out simultaneously on a fiber drawing tower.
The resulting capillary tubes are then assembled in a bundle around the central rod 15 in the preform. The drawing conditions, such as the drawing temperature and speed, enable the ratio φint/φext to be modified and the final pitch Λ between the holes to be fixed.
Depending on the application, the central rod 15 can be of pure or doped silica, for example silica doped with dopants such as Ge, P, Al, La, Ga, Nb, Li or rare earth dopants such as Er, Yb, Ce, Tm, Nd.
The FIG. 2 graph shows the attenuation in a fiber with holes obtained by a method as previously described. The spectral attenuation AS(λ), expressed in dB per kilometer, is deduced from the following expression:AS(λ)=10 log[PL(X)−Pl(λ)]/(l−L)]                in which:        PL is the power measured at the end of a “long length” L of fiber, and        Pl is the power measured at the end of a “short length” l of fiber, corresponding to one end of the section L.        
Note the occurrence of high attenuation peaks at 950 nm, 1250 nm and 1390 nm, which are directly related to the presence of hydroxyl (OH) groups. This is because, during the fabrication process, hydroxyl groups form and are absorbed at the interfaces of the capillary tubes, leading to pollution of the fiber.
This pollution is well known to fiber fabricators, but in conventional fibers it is controlled and limited to a low level, in particular because the air-silica interface is far away from the area in which the signal propagates. On the other hand, in fibers with holes, the air-silica interfaces in contact with the propagation signal are multiplied, which causes high absorption peaks in the range of operating wavelengths.
The object of the present invention is to reduce the contribution of the peaks associated with the presence of hydroxyl groups in a fiber with holes.
To this end, the invention proposes to produce a self-cleaning layer within the capillary tubes that is adapted to react with the hydroxyl groups to evacuate them in the form of volatile substances.